This invention relates to a video signal processor for achieving frame or field rate conversion.
In television standards converters there is a requirement to change the number of fields or frames per second from one standard rate to another, or to change the number of lines per field or frame from one standard rate to another, or to change both. One requirement which arises frequently is that of changing from a 525 lines per frame/60 fields per second standard to a 625 lines per frame/50 fields per second standard, or vice versa, both standards having a 2:1 interlace. The present invention is concerned with a processor for changing the field or frame rate.
FIG. 1 shows a selection of fields (a) to (d) reproduced from a sequence of video signals having a 60 Hz field rate and 2:1 interlacing, and a selection of fields (e) to (g) from video signals representing the same scene, but having a 50 Hz field rate, also with 2:1 interlacing. The scene is shown as including an object (h) which moves to take up different positions in successive fields. Periodically one of the 60 Hz fields and one of the 50 Hz fields substantially correspond, so as to represent the scene at substantially the same time. Two such `coincident` fields are depicted at (a) and (e) and the next two at (d) and (g). If the fields (a) and (e) are both odd line fields, then (assuming the 60 and 50 Hz field rates as stated) the fields (d) and (g) are odd and even line fields respectively, the next two coincident fields are again odd line fields and so on.
Between the pairs of coincident fields, each 50 Hz field represents the scene at a time between two 60 Hz fields, the time differentials changing from field to field in a regular manner. This is illustrated by the two 60 Hz fields (b) and (c) and the 50 Hz field (f) which represents the scene at a time between (b) and (c). In field (f) the moving object should occupy a position between the positions occupied in fields (b) and (c). However, no video signals are available in the nearest 60 Hz field periods, positioned correctly to represent the object (h) as required for field (f).
This kind of difficulty is encountered generally in field rate conversion and it is aggravated by the fact that when generating, say, an odd line field at the converted rate the incoming field with the least positional error may be either an odd line field or an even line field. One approach to the problem is merely to produce signals of the desired field rate by selecting for each outgoing field signals from the incoming fields for which the time differential is least. However, if there is substantial movement in the scene the movement becomes disturbingly uneven after the conversion due to the unequal time differentials.